1. Field of the Invention
The present invention relates to an information recording medium for writing (i.e., recording) and/or reading out (i.e., reproducing) information by means of a laser beam having high energy density.
2. Description of Prior Art
Information recording media for recording and/or reproducing information by the use of a laser beam have been developed in recent years and are put to practical use. Such recording media have been widely utilized in various fields, for example, as an optical disc such as a video disc and an audio disc as well as a disc memory for a large-capacity computer and a large-capacity static image file, a micro-image recording medium, a ultra-micro-image recording medium, a micro-facsimile, and an optical card.
The conventional optical information recording medium basically comprises a transparent substrate of a plastic or glass material and a recording layer provided on the substrate. As materials of the recording layer, metals such as Bi, Sn, In and Te or semimetals, and dyes such as a cyanine dye, a metal complex compound and a quinone dye are generally known.
Writing (i.e., recording) of information on the recording medium can be conducted, for example, by irradiating the medium with a laser beam. Under irradiation with the laser beam, the irradiated area of the recording layer of the recording medium absorbs energy of the beam and rise of temperature locally occurs and, as a result, a chemical or physical change is caused to alter(or change) optical characteristics of the recording layer in the irradiated area, whereby the recording of information can be made. Reading of the information from the recording medium is also conducted by irradiating the medium with a laser beam. The information can be reproduced by detecting reflected light or transmitted light corresponding to the change in the optical characteristics of the recording layer.
Recently, an optical disc having an air-sandwich structure for protecting the recording layer has been proposed. The optical disc of air-sandwich structure comprises two disc-shaped substrates, a recording layer provided on at least one of the substrates and two ring-shaped spacers (inner spacer and outer spacer), said two substrates interposing the recording layer being combined with each other in such a manner that a closed space is formed by the two substrates and the two spacers. In such recording medium, the recording layer is kept from direct exposure to an outer air, and recording or reproduction of information is carried out by applying light of the laser beam to the recording layer through the substrate, whereby the recording layer is generally kept from physical or chemical damage. Further, the surface of the recording layer can be kept from deposition of dust which likely causes troubles in the recording and reproducing procedures.
As mentioned above, inorganic materials such as metals and semimetals or dyes have been conventionally used for forming the recording layer. For example, Japanese Patent Provisional Publication No. 58(1983)-112790 discloses that a cyanine dye or a merocyanine dye is used alone as a recording material of the recording medium. However, most of those dyes generally have a peak (.lambda. max) of the reflection spectrum at approx. 600 mn, and this peak is on the shorter wavelength side than the wavelength of a semiconductor laser (generally at approx. 800 nm) which is generally used in the recording or reproduction of information for the recording medium, so that the reflectance of the resulting recording layer is low and a S/N ratio thereof is in the recording procedure is not high. For shifting the peak wavelength (.lambda. max) of the reflection spectrum to a longer wavelength side, it is thought that the ethylene chain of the dye compound is prolonged, that is, n in (CH.dbd.CH).sub.n is made larger. In this case, however, the resulting recording layer comprising the dye compound deteriorates in durability (endurance), especially in the durability in the reading procedure (i.e., resistance to light).